Methods of storing snapshot data

The Copy-After-Write (CAW) method is the default method of storing snapshot data. However, if the write pending rate (percentage of data waiting to be written in cache memory) is 60% or higher when writing in cascaded pairs random access mode, or 30% or higher when writing in sequential access mode, the Copy-On-Write (COW) method is used.

In the CAW method, writing the P-VOL snapshot data changes the status to “write completion”. In the COW method, storing the P-VOL snapshot data changes the status to "write completion". The CAW method wait time is shorter than that of the COW method.

Workflow for the CAW method

The following workflow describes the CAW method and how Virtual Storage Platform 5000 series stores snapshot data:

1. The host writes data to a P-VOL.
2. The storage system returns the write completion status to the host.
3. The storage system stores snapshot data for the P-VOL in the background.

The following figure illustrates the CAW method.

Workflow for the COW method

The following workflow describes the COW method and how Virtual Storage Platform 5000 series stores snapshot data:

1. The host writes data to a P-VOL.
2. The storage system stores snapshot data for the P-VOL.
3. The storage system returns the write completion status to the host.

The following figure illustrates the COW method.